In typical NMR spectrometers, a sample is placed in a region of uniform magnetic field, for example, between pole pieces of a magnet assembly. A wire coil and electronic circuits are provided to both apply radio-frequency (RF) pulses to the sample (transmit) and to detect RF signals from the sample (receive). In a typical experiment or measurement, the protons (or other nuclei under study) in the sample are first allowed to polarize in the magnetic field. Establishing equilibrium polarization takes a time which is known as T1 in the NMR art. This time ranges from a fraction of a second up to 20 seconds, depending on the composition of the liquid sample. Next, one or more RF pulses are applied to the sample with frequencies at or near the resonance frequency at which the protons freely precess in the applied uniform magnetic field. The transmit pulses have the effect of tilting the proton polarization relative to the direction of the applied field. After the transmit pulse is ended the protons precess and create a time-varying magnetic field in the coil. The time-varying field induces a signal voltage in the coil which may be amplified and recorded. Either immediately, or after a delay time, the receive signal is recorded and transferred to a computer for processing.
A particular sequence of transmit pulses, delay times and data acquisition periods, together with all of the parameters needed to precisely define the sequence (times, frequencies, phases, acquisition rates) is know in the NMR art collectively as a pulse sequence. The design of NMR pulse sequences to extract desired information from a sample is a well-developed area of knowledge. A great variety of pulse sequences are known in the art, ranging from simple ones with a single transmit pulse and a single data acquisition time to much more complex sequences with many transmit pulses, time delays, transmitter phase and amplitude adjustments, and data acquisition periods. The goals of these sequences vary widely, including enhancement of spectroscopic resolution, enhancement of signal-to-noise ratio, detection of spin-spin coupling between different groups of protons (or other nuclei), suppression of strong solvent signals, suppression of coupling between like or unlike nuclei, transfer of polarization from one group of nuclei to another, measurement of relaxation rates, measurement of diffusion within the fluid and measurement of fluid flow.
The foregoing examples of related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those skilled in the art upon a reading of the specification and a study of the drawings.